The present invention relates to polymeric composites of rigid-rod aromatic heterocyclic polymers and flexible thermoplastic polymers and a method for the preparation of composite articles with rigid-rod aromatic heterocyclic polymers molecularly dispersed in a continuous thermoplastic polymeric matrix.
Polybenzobisazole polymers, and particularly the so-called "rigid-rod" benzobisazole polymers, are noted for high tensile strength, high tensile modulus and high thermal stability. However, many benzazole polymers are difficult to fabricate into useful articles. Rigid and semi-rigid polybenzobisazoles do not have glass transition temperatures at any temperature at which they are stable. Therefore, the polymers are ordinarily spun from solution to form fibers, which serve as reinforcement within a thermosetting matrix, such as epoxy resins, to form composites. However, the fibers and the cured composites are not moldable or thermoformable.
Many moldable and thermoformable polymers are known. Exemplary polymers include thermoplastic polyamides, polyimides, polyquinolines, polyquinoxalines, poly(aromatic ether ketones) and poly(aromatic ether sulfones). However, those polymers do not have the high tensile strength and modulus which are characteristic of polybenzobisazole polymers.
Attempts have been made to synthesize articles which combine the processability of the thermoplastic polymer with the superior mechanical properties of the polybenzobisazole polymers. To this end, molecular composites of rigid rod polybenzobisazole and flexible polymers have been studied. A molecular composite is defined as a molecular dispersion of a rigid-rod polymer in a flexible polymer matrix such that the rigid-rod polymer acts as the reinforcing elements. To ensure molecular dispersion of the rigid-rod polymer in the blend, processing of the composite can only be carried out from solution at a concentration lower than the critical concentration, C.sub.cr, to prevent segregation of the rigid-rod reinforcement. At a concentration above C.sub.cr, the solution becomes biphasic where the rigid-rod segregates into liquid crystalline domains which are dispersed in an entangled flexible matrix. Such molecular composites are described in numerous references, such as Helminiak et al, U.S. Pat. No. 4,207,407, issued Jun. 10, 1980; Helminiak et al, U.S. Pat. No. 4,377,546, issued Mar. 22, 1983; Hwang et al, U.S. Pat. No. 4,631,318, issued Dec. 23, 1986; Nishihara et al, U.S. Pat. No. 4,749,753, issued Jun. 7,1988; Uy, U.S. Pat. No. 4,972,028, issued Nov. 20, 1990; Arnold et al, U.S. Pat. No. 4,977,223, issued Dec. 11, 1990; Tan et al, U.S. Pat. No. 5,086,120, issued Feb. 4, 1992; Arnold et al, U.S. Pat. No. 5,095,075, issued Mar. 10, 1992 and Hwang et al, U.S. Pat. No. 5,217,809, issued Jun. 8, 1993.
However, polybenzobisazoles, and particularly rigid-rod polybenzobisazoles, are incompatible with many thermoplastic polymers. When dopes containing polybenzobisazole and a thermoplastic polymer are coagulated, the thermoplastic agglomerates and/or phase separates. The resulting shaped articles either have poorer properties in all directions than the corresponding thermoplastic alone, or have superior properties in one direction and inferior properties in all other directions. Such compositions may be useful for fibers, but not for molded articles.
What are needed are materials and processes which can be used to make molded articles containing reinforcing amounts of polybenzobisazole polymer which have superior properties in at least two dimensions and/or are not substantially phase separated.
Accordingly, it is an object of the present invention to provide molecular dispersions of rigid-rod polybenzobisazole polymer in a thermoplastic matrix.
It is another object of the present invention to provide molecular dispersions of rigid-rod polybenzobisazole polymer in a thermoplastic matrix above the critical concentration to a degree at which the polymer blend can be cast from solution.
It is yet another object of the present invention to provide a method for preparing molecular dispersions of rigid-rod polybenzobisazole polymer in a thermoplastic matrix above the critical concentration to a degree at which the polymer blend can be cast from solution.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following detailed disclosure of the invention.